Deepwater accumulators provide a supply of pressurized working fluid for the control and operation of subsea equipment, such as through hydraulic actuators and motors. Typical subsea equipment may include, but is not limited to, blowout preventers (BOPs) that shut off the well bore to secure an oil or gas well from accidental discharges to the environment, gate valves for the control of flow of oil or gas to the surface or to other subsea locations, or hydraulically actuated connectors and similar devices. Accumulator fluid power may be used to operate underwater process valves and connectors, as well as supply of non-continuous process chemicals into a process stream at the seafloor. Applications may also include management of fluid power and electrical power on subsea drilling BOP stacks, subsea production Christmas trees, workover and control systems (WOCS), and subsea chemical injection systems.
Accumulators are typically divided vessels with a gas section and a hydraulic fluid section that operate on a common principle. The principle is to precharge the gas section with pressurized gas to a pressure at or slightly below the anticipated minimum pressure required to operate the subsea equipment. Fluid can be added to the accumulator in the separate hydraulic fluid section, increasing the pressure of the pressurized gas and the hydraulic fluid. The hydraulic fluid introduced into the accumulator is therefore stored at a pressure at least as high as the precharge pressure and is available for doing hydraulic work.
Accumulators generally come in three styles—the bladder type having a balloon type bladder to separate the gas from the fluid, the piston type having a piston sliding up and down a seal bore to separate the fluid from the gas, and the float type with a float providing a partial separation of the fluid from the gas and for closing a valve when the float approaches the bottom to prevent the escape of the charging gas. A fourth type of accumulator is pressure compensated for depth and adds the nitrogen precharge pressure plus the ambient seawater pressure to the working fluid.
The precharge gas can be said to act as a spring that is compressed when the gas section is at its lowest volume/greatest pressure and released when the gas section is at its greatest volume/lowest pressure. Accumulators are typically precharged in the absence of hydrostatic pressure and the precharge pressure is limited by the pressure containment and structural design limits of the accumulator vessel under surface ambient conditions. Yet, as accumulators are used in deeper water, the efficiency of conventional accumulators decreases as application of hydrostatic pressure causes the gas to compress, leaving a progressively smaller volume of gas to charge the hydraulic fluid. The gas section must consequently be designed such that the gas still provides enough power to operate the subsea equipment under hydrostatic pressure even as the hydraulic fluid approaches discharge and the gas section is at its greatest volume/lowest pressure.
For example, accumulators at the surface typically provide 3000 psi working fluid maximum pressure. In 1000 feet of seawater the ambient pressure is approximately 465 psi. For an accumulator to provide a 3000 psi differential at 1000 ft. depth, it must actually be precharged to 3000 psi plus 465 psi, or 3465 psi.
At slightly over 4000 ft. water depth, the ambient pressure is almost 2000 psi, so the precharge would be required to be 3000 psi plus 2000 psi, or 5000 psi. This would mean that the precharge would equal the working pressure of the accumulator and any fluid introduced for storage may cause the pressure to exceed the working pressure and accumulator failure.
At progressively greater hydrostatic operating pressures, the accumulator thus has greater pressure containment requirements at non-operational (no ambient hydrostatic pressure) conditions.
The accumulator design must also take into account human error contingencies. For example, removal of the external ambient hydrostatic pressure without evacuating the fluid section of the accumulator to reestablish the original gas section precharge pressure may result in failure due to gas section pressures exceeding the original precharge pressures.
Accumulators may be included, for example, as part of a subsea BOP stack assembly assembled onto a subsea wellhead. The BOP assembly may include a frame, BOPs, and accumulators to provide back up hydraulic fluid pressure for actuating the BOPs. The space available for other BOP package components such as remote operated vehicle (ROV) panels and mounted controls equipment becomes harder to establish due to an increasing number and size of the accumulators required to be considered for operation in deeper water depths. The accumulators are also typically installed in series where the failure of any one accumulator prevents the additional accumulators from functioning.
The inefficiency of precharging accumulators under non-operational conditions thus requires large aggregate accumulator volumes that increase the size and weight of the subsea equipment. Yet, offshore rigs are moving further and further offshore to drill in deeper and deeper water. Because of the ever increasing envelope of operation, traditional accumulators have become unmanageable with regards to quantity and location. In some instances, it has even been suggested that in order to accommodate the increasing demands of the conventional accumulator system, a separate subsea skid may have to be run in conjunction with the subsea equipment in order to provide the required volume necessary at the limits of the water depth capability of the equipment. With rigs operators increasingly putting a premium on minimizing size and weight of the drilling equipment to reduce drilling costs, the size and weight of all drilling equipment must be optimized.